Vast quantities of cellulosic materials occur universally as surplus and waste residues of industrial processing and other operations. They occur in various processed forms, such as, pulp sludges from the manufacture of paper, commonly known as clarifier sludge, coffee, commonly known as coffee grounds, and cane sugar, commonly known as bagasse pith. Basically, these materials contain cellulose, usually in combination with significant amounts of hemicellulose and smaller amounts of lignin. Because of their carbohydrate content, these materials represent potentially valuable biorenewable resources for fermentation processes in the production of edible microbial protein.
Various known processes have been proposed or used to convert cellulosic materials into products which are purported to be suitable, as substitute for hay and similar forages, for metabolisable carbohydrate energy in ruminant feeds. These processes use physical and/or chemical changes of the materials merely to enhance their digestibility; none of the original cellulosic material is converted into protein.
Various known processes have also been proposed or used to convert cellulosic materials into products which are purported to be suitable, as substitutes for soymeal and similar protein-rich substances, for animal feed protein rations. Of these, two basic types of fermentation processes are known. In the first method, yeasts are cultivated on liquid sugar solutions which are produced by chemical hydrolysis of the hemicellulose and/or cellulose components of the cellulosic material. In the second method, cellulolytic bacteria are cultivated on the solid cellulose and/or hemicellulose components of the cellulosic material.
In the above prior art processes, one or more of the following undesirable features are found: (1) the processing costs are high, because conversion rates are low and/or chemical pretreatment, usually under severe conditions, of the material is required, (2) the product is not suitable as animal feed, because it is too toxic and/or too indigestible, (3) the product is not suitable as animal feed protein ration, because its protein content is too low and/or its protein quality is too poor.
In a publication by the inventor and his co-workers entitled "SCP Production by Chaetomium cellulolyticum, a new Thermotolerant Cellulolyticum Fungus", Biotechnology and Bioengineering, vol. XlX, pages 527 to 538 (1977), it is disclosed that Chaetomium cellulolyticum showed 50 to 100% faster growth rates and over 80% more final biomass-protein formation than Trichoderma viride when cultivated on highly purified cellulose as the sole carbon source in the fermentation media.
This article also discloses that for other forms of cellulose (namely, sawdust) extensive pretreatment is required to render the material susceptible to use for fermentation.